Re: [Qemu-devel] [PATCH v4 2/3] linux-aio: handling -EAGAIN for !s->io_q.plugged case

[Ming Lei <ming.lei@canonical.com>]

On Mon, Nov 24, 2014 at 7:38 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
>
> On 24/11/2014 12:31, Ming Lei wrote:
>> +    /* don't submit until next completion for -EAGAIN of non plug case */
>> +    if (unlikely(!s->io_q.plugged)) {
>> +        return 0;
>> +    }
>> +
>>      /* submit immediately if queue depth is above 2/3 */
>>      if (idx > s->io_q.size * 2 / 3) {
>>          return ioq_submit(s);
>       }
>
>       return 0;
>
> so I fail to see why my proposal:
>
>
>     /* This is reached in two cases: queue not plugged but io_submit
>      * returned -EAGAIN, or queue plugged.  In the latter case, start
>      * submitting some I/O if the queue is getting too full.  In the
>      * former case, instead, wait until an I/O operation is completed.
>      */
>     if (likely(idx <= s->io_q.size * 2 / 3) || unlikely(!s->io_q.plugged) {
>         return 0;
>     }
>
>     return ioq_submit(s);
>
> was wrong.  Can you explain?

I didn't say your proposal is wrong, and this patch is correct too
without fat comment.

The difference is only that this patch returns immediately in case
of !s->io_q.plugged after putting the req into io queue.

Thanks,